Mary/Adams on fri 12 aug 05
Once again, my 40 or so books aren't helping me answer this question. I
don't really know how the cone number correlates with what the clay is made
of. I know I've asked the question whether it would be detrimental to knead
one cone clay on a table where you've just kneaded another and several of
you told me that trivial amounts wouldn't matter. But, what is it that
makes a clay Cone 5 vs Cone 10?
m
Craig Clark on sat 13 aug 05
Mary/Adams wrote:
>Once again, my 40 or so books aren't helping me answer this question. I
>don't really know how the cone number correlates with what the clay is made
>of. I know I've asked the question whether it would be detrimental to knead
>one cone clay on a table where you've just kneaded another and several of
>you told me that trivial amounts wouldn't matter. But, what is it that
>makes a clay Cone 5 vs Cone 10?
>
>m
>
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Mary, I believe that would be the silica to alumina ratio in the
body which determines the point of vitrification. I also agree that
wedging a cone 10 body or a cone 6 body on the same surface isn't really
an issue. I go back and forth between cone 6 bodies and raku bodies. No
problems. More of an issue would be trying to wedge a light or white
colored clay body on the same surface that you have just been wedging a
darker clay body on. Though this is only an issue if you are using
canvas covered wedging surfaces. Take a look at the archives. There have
been some pretty good discussions about work surfaces in the studios
lately. There has been a very good argument made to basically strip the
old canvas off the surfaces and use sealed surfaces to work on that are
cleaned after each use.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
Ron Roy on sun 14 aug 05
Hi Mary,
It seems simple to me - perhaps gecauseI have been "into" it for so long.
Let me know if this helps.
It's all about melting silica and alumina - so this applies to glazes as well.
Lets say you want to have your clay have a water absorption of about 2% -
so it will not leak.
At cone 6 you need to add more flux for that that to happen - less at cone 10.
If you calculate the Seger Unity formula for a cone 6 clay and one that
works properly at cone 10 you will see - the cone 10 formula will show more
silica and alumina.
Need more?
RR
>Once again, my 40 or so books aren't helping me answer this question. I
>don't really know how the cone number correlates with what the clay is made
>of. I know I've asked the question whether it would be detrimental to knead
>one cone clay on a table where you've just kneaded another and several of
>you told me that trivial amounts wouldn't matter. But, what is it that
>makes a clay Cone 5 vs Cone 10?
>
>m
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Snail Scott on tue 16 aug 05
At 03:34 PM 8/12/2005 -0700, you wrote:
>...I
>don't really know how the cone number correlates with what the clay is made
>of...But, what is it that
>makes a clay Cone 5 vs Cone 10?
All clays are made mostly of silica and alumina,
in varying percentages depending on the mineral
deposit it was mined from. These two minerals by
themselves won't melt at any reasonable kiln
temperature. But, most clays also have other
minerals in them, including potassium, sodium,
calcium, and other minerals which DO melt at
resonable temperatures. These are called fluxes.
These minerals help the silica and alumina melt,
and fuse everything together into a ceramic
material when fired hot enough.
Some fluxes melt at lower temperatures than others,
and a clay which contains a large percentage of
these will be a lower-firing clay than one which
has hotter-melting fluxes, and/or a smaller amount
of the same fluxes.
Cone 5 and cone 10 are really pretty close together,
temperature-wise, so the same fluxes work pretty
well for both temperatures. So, the main difference
is that a ^5 clay has more of those fluxes than
a ^10 clay, and so it melts ('vitrifies') at a
somewhat lower temperature.
Clays, as I mentioned, are dug out of mines, and vary
depending on the mineral content of that mine. It's
rare, though, that a clay will be just right for a
particular ceramic purpose. We might wish it fired
hotter, or was more squishy ('plastic'), or grittier,
or a different color, or whatever. So, that clay gets
mixed with other clays from other mines, and with
other minerals like feldspars (which contain lots of
fluxes,) to make a mixture that behaves the way we
prefer. This mixture is called a 'clay body'. If you
buy premixed wet clay form a ceramic supply shop,
what you are getting is a clay body.
The manufacturer comes up with recipes for clay
bodies, based on what they think their customers will
want, and sell the various clay bodies made from those
recipes. So, for instance, if they decide to make a
clay body designed to throw large pottery well, that's
red, and vitrifies at ^10, they might choose to use
some fire clay to help bring the firing temperature up
high, some ball clay, which makes it plastic for easy
throwing, some red clay or iron to give a nice warm
color to the stuff, and some sand to help it hold its
shape when thrown large, and some feldspar to help it
melt well.
Now, say that the manufacturer has customers that want
to save energy by firing lower, but they want the same
properties as that ^10 clay. The manufacturer will
probably start with the same recipe, but add extra
feldspar (or an additional type of feldspar) because
the feldspar contains lots of potassium or sodium,
and by adding more of those elements, the clay body
will melt more thoroughly at a lower temperature. By
adjusting the amount of flux-bearing ingredients like
the feldspar, the manufacturer can fine-tune the
firing temperature to one desired cone, like ^5. If
that new ^5 clay body were to be fired to ^10, all
that extra flux would melt it onto a blob, or at least
cause it to warp and slump as the silica and alumina
in the clay became more and more dissolved in the
fluxes.
When someone decides to mix their own clay body, they
simply go thrugh the same procedure, choosing the
clays and minerals that will give the properties that
they like in a clay, including the amount of flux
minerals. It's the fluxes that control the temperature
that a clay will vitrify at, whether those fluxes are
naturally occurring within the clays used, or are
added as separate minerals into the clay body.
Cone numbers, by the way, are an arbitrary scale
devised by a German researcher named Hermann Seger,
back in the 19th century. He invented pyrometric cones,
making them from specific percentage ratios of silica,
alumina, and flux. He picked a starting point which
was an easy ratio, called that "cone 1", and deducted
a fixed percentage of flux from each subsequent cone:
cone 2, cone 3, cone 4, and so on, each one vitrifying
at at hotter temperature than the cone before it.
(The cones starting with '0', which read like negative
numbers going down in temperature, were worked out
later.) That's why cones don't correlate in a simple
way with thermometer temperatures. They were designed
from a chemist's starting point. So you can see, it's
no accident that cone numbers and flux percentages are
really just two sides of the same coin. Lower amount
of flux = higher cone number and higher vitrification
temperature.
-Snail
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